printedparts stage 8: 1x0.5in bin labels

New public print view at /print/printedparts-labels following the
plugin-owned USB label precedent: multi-select with per-item copies,
CODE128 of the item code via JsBarcode (a QR at this size is at the
edge of scanner tolerance), one label per page on 1in x 0.5in roll
stock via a new @page size. The Detail page's Bin Label button
preselects its item through ?item=<id>; the list header gains a batch
Print Labels button.

frontend/src/views/printedparts/PrintedItemsList.vue

@@ -2,7 +2,12 @@
   <div>
     <div class="page-header">
       <h2>3D Printed Parts</h2>
-      <router-link to="/printedparts/new" class="btn btn-primary">Add Part</router-link>
+      <div class="header-actions">
+        <router-link to="/print/printedparts-labels" class="btn btn-secondary">
+          Print Labels
+        </router-link>
+        <router-link to="/printedparts/new" class="btn btn-primary">Add Part</router-link>
+      </div>
     </div>
 
     <div class="filters">
@@ -131,6 +136,7 @@ function debouncedSearch() {
   text-overflow: ellipsis;
   white-space: nowrap;
 }
+.header-actions { display: flex; gap: 0.5rem; }
 .lowstock-filter {
   display: inline-flex;
   align-items: center;